Kelly, who is running as a Democrat, retired from NASA in 2011 after completing several missions for the space agency, including one mission where he spent more than 50 days in space and lived on the International Space Station. He is married to former Rep. Gabby Giffords, who survived a shooting in 2011. The couple appeared in the announcement video together, and talked about that period in their lives while Giffords recovered from the gunshot wound.

"I learned a lot from being an astronaut. I learned a lot from being a pilot in the Navy. I learned a lot about solving problems from being an engineer," Kelly says in the video. "But what I learned from my wife is how you use policy to improve people's lives."

In the campaign announcement video, Kelly highlights several key issues he cares about and says he will work on if elected. They include climate change, affordable health care, wage stagnation, job growth and the economy.

Kelly is running for the late Republican Sen. John McCain's old seat. McCain won re-election in 2016, but after he passed away from brain cancer in August 2017, former Arizona Rep. Martha McSally was appointed by the governor to fill the seat until the 2020 special election.